Large recovery boilers used in pulp and paper mills typically include a densely packed array of generating bank or boiler tubes. Each tube has an open end in communication with a steam or water drum. It has previously been recognized that some boiler tubes are most susceptible to corrosion at their point of attachment to the drum. Accordingly, various ultrasonic probes have been proposed in the past for determining the wall thickness of the boiler tubes in that region.
U.S. Pat. No. 4,353,257 granted to Vrba et al on Oct. 12, 1982 discloses a system for periodically inspecting the thickness of boiler tubes using an ultrasonic probe. The Vrba probe assembly contains a transducer which is mechanically coupled to a motor drive. Rotation of the motor drive causes the transducer to move in a helical path into and out of the boiler tube to be tested. The Vrba probe assembly further includes a carriage which is urged against the tube inner wall by a spring. The purpose of the carriage is to maintain the transducer in the optimum orientation relative to the tube inner wall.
The primary drawback of the Vrba device is that it would not be suitable for accurately measuring corroded tube areas which are not parallel to the centerline of the tube. As a result of the construction of the transducer carriage, the Vrba probe is not optimized for use in tubes having a rolled or undulating contour. The scanning path is entirely controlled by the motor drive and the probe itself is not manually manipulatable.
U.S. Pat. No. 4,663,727 granted to Saporito et al on May 5, 1987 is also exemplary of the prior art. This reference discloses an ultrasonic inspection system employing a transducer mounted at one end of an elongate probe connected to a rotational and axial drive mechanism. No means are provided for manually skewing the probe out of alignment with the longitudinal axis of the tube to be inspected to detect corroded areas not parallel to the centerline of the tube.
U.S. Pat. No. 4,412,315 granted to Flournoy on Oct. 25, 1983 discloses an acoustic pulse-echo wall thickness method and apparatus for detecting anomalies, such as pits, in pipelines. The apparatus includes a transducer capable of generating simultaneous acoustic pulses in opposite directions and two reflectors for reflecting the acoustic pulses toward the inner wall of the pipe under inspection. One of the pulses is directed in a path normal to the pipe wall and the other is reflected in a path oblique to the pipe wall. The Flournoy device is thus capable of measuring wall thickness and simultaneously determining the presence of anomalies in the wall being surveyed. However, it suffers from the disadvantage that ultrasonic pulses may be directed at the pipe inner wall only at a single oblique angle, as determined by the orientation of the reflector face. If it was desired to direct scanning pulses at a plurality of oblique angles, then a plurality of transducers and reflector surfaces would be required.
Accordingly, the need has arisen for a hand-held ultrasonic probe incorporating a single transducer which may be manually skewed relative to the longitudinal axis of the tube to direct ultrasonic acoustic pulses at a plurality of oblique angles relative to the tube inner wall and thereby accurately identify corroded tube areas which are not parallel to the longitudinal axis of the tube.